Atomic force metrology and 3D modeling of microtrenching in etched photomask features

With the acceptance of etched phase shift photomasks by most major semiconductor manufacturers, it is necessary to build a significant number of these masks in a cost-effective and controlled manner. Optical methods of metrology used for many years in the photomask industry for binary masks are unsuitable for certain metrology applications related to etched phase shift photomask manufacture and repair. Atomic Force Metrology (AFM), Scanning Electron Microscope (SEM) metrology and optical methods, in combination, are now providing the necessary metrology tools to characterize etched phase shift masks and generate metrology for phase defect repair. Modeling methods such as electromagnetic modeling (EM) are providing insight into the effect of non- ideal processing on the final printed wafer image and the aerial image presented by a lithography system. This paper uses AFM metrology and EM modeling to explore the details of micro-trenching that occurs at the base of vertical walls on etched phase shift masks. Results of etch profiles obtained using AFM metrology are correlated with EM modeling of 320nm - 720nm etched structures in alternating aperture phase shift masks for use with KrF 4x reduction steppers. Three generically different etch processes were used to create the measured structures on six different masks. These six masks were manufactured by four different mask makers. The metrology and EM modeling results clearly differentiate each of the generic etch processes with the ICP + Iso etches showing 50% less micro-trenching relative to three of the four RIE-only etch processes. The simulation predicts the process window changes resulting from modeled micro-trenching relative to an ideal etch case. The models chosen for simulation represent the real etched results observed with the AFM. Comparisons of micro-trenching lengths measured during these experiments with the results of trenching bias across different etched space widths as reported by McCallum, et al. suggest that micro-trenching is a universal phenomenon occurring during quartz etches. It is proposed that micro- trenching more correctly defines the geometries responsible for trenching bias. Depth detail taken from the vicinity of an unusually shaped phase bump defect show the effect of local geometry on the extent to which trenching occurs. Trenching is more than 50% greater at the base of concave or acute defects as compared to convex defects. Further investigation of local trenching sheds light on the reason why phase bump defects near or touching a vertical trench wall appear more difficult to repair.